Filtering apparatus for a drop writing system

ABSTRACT

A filter for a drop writing apparatus positioned adjacent to an entrance end of a nozzle fluid resonant cavity which has at its opposite end a fluid ejection orifice of the order of 0.001 inch to 0.0025 inches in diameter. The filter has a microscopic porosity capable of preventing passage through of particles of the order of one-half the diameter of the ejection orifice, yet is mounted in an entrance chamber having a predetermined diameter relationship to the cavity diameter so as to have substantially no effect on the fluid resonance of the cavity.

United States Patent 1 1 Keur [541 FILTERING APPARATUS FOR A DROP WRITING SYSTEM [75] Inventor: Robert 1. Kent, Niles, 111.

[73] Assignee: A. B. Dick Company, Chicago, 111.

[22] Filed: April 19, 1971 [21] Appl. No.: 135,312

[52] US. Cl. ..239/1, 239/102, 222/189, 239/5903, 346/75 [51] int. Cl. ..A26c 1/06, B051) 17/00, 844d H08 [58] Field of Search...239/10l, 102, 590, 590.3, 337; 222/189; 346/75 [56] References Cited UNITED STATES PATENTS 3,281,859 10/1966 Stone ..239/102 X 3,149,758 9/1964 Bush et al...... ..222/l89 3,362,640 1/1968 Fainman ..239/1 2,512,743 6/1950 l-lansell .....239/101 X 3,287,734 11/1966 Kazan ..346/75 460,368 9/1891 Splittstoser 1,835,865 12/1931 Hansen 2,355,550 8/1944 Nusbaum ..239/337 Primary Examiner-Lloyd L. King Attorney-Peter S. Lucyshyn [57] ABSTRACT A filter for a drop writing apparatus positioned adjacent to an entrance end of a nozzle fluid resonant cavity which has at its opposite end a fluid ejection orifice of the order of 0.001 inch to 0.0025 inches in diameter. The filter has a microscopic porosity capable of preventing passage through of particles of the order of one-half the diameter of the ejection orifice,

-yet is mounted in an entrance chamber having a predetermined diameter relationship to the cavity diameter so as to have substantially no effect on the fluid resonance of the cavity.

4 Claims, 3 Drawing Figures P'A'IENTEDJM 2191a 3.708.118

SHEET 1 BF 2 FIG. 1

INVENTOR ROBERT I. KEUR PATENTEU 21975 3.708.118

SHEET 2 0F 2 Dl -D2 FIG.3 0.2

FLUID RESONANCE FREQUENCY INVENTOR ROBERT I. KEUR FILTERING APPARATUS FOR A DROP WRITING SYSTEM The invention relates to apparatus for producing information represented by electrical signals using writing fluid drops deposited on a writing medium, and more particularly to an improvement in the fluid flow system.

BACKGROUND OF THE INVENTION Apparatus has been developed to make a record on a writing medium of the information represented by video signals by generating a stream of writing fluid drops, directing these writing fluid drops toward a writing medium and then deflecting these drops in their travel toward the writing medium in response to video signals in a manner so that when the drops do reach the writing medium a visible image is represented of them. The apparatus which is employed usually comprises a writing fluid reservoir which provides a supply of writ-' ing fluid for a pump. The output of the pump is fed to a nozzle which comprises a part of a print head assembly. As has been described in Stone, U.S. Pat. No. 3,346,869, assigned to A. B. Dick Company, Assignee of the present invention, the nozzle orifice is very small, having a diameter in the range of 0.001 inch to 0.0025 inch. As a result there are problems with clogging of the nozzle. The latter patent is directed to solving the nozzle clogging problem resulting from writing fluid drying in the nozzle orifice.

Another clogging problem occurs from particles present in the writing fluid. The drop writing systems of I the prior art try to solve the problem by providing a filter assembly in the fluid line between the writing fluid pump and the nozzle. However, there remains a substantial length of tubing and connecting parts between the filter assembly and the nozzle orifice. The reason is that the filter assembly is bulky and locating it on the nozzle assembly which moves back and forth across the writing medium, would deleteriously affect the response of the print head. Furthermore, it was believed that an accoustically soft terminus, such as flexible tubing, was necessary at one end of the nozzle to avoid changing the fluid resonance properties of the nozzle.

Yet experience has shown that by having a filter down line from the nozzle, minute particles are trapped in the flow system between the filter and the nozzle. These particles may be scrapings formed as the parts are assembled, or they may enter the system in some other way during assembly. Even with the use of protective environments and careful procedure during fitting of the parts together there still are minute particles which are downstream from the filter that cause nozzle clogging and failure in the system.

Furthermore, certain physical properties of the nozzle assembly are critical for obtaining desirable operation. That is explained in a co-pending application, Keur et al., Ser. No. 61,111, filed Aug. 8, 1970, assigned to A. B. Dick Company, the Assignee ofthe present application. Because the filters contemplated by the present invention are intended to remove particles of the order of microns (one micron equals about 0.000039 inch), the filter is substantially a solid wall to accoustic waves. For this reason it is not a simple matter of placing a filter near the nozzle orifice as shown in Fainman, U.S. Pat. No. 3,362,640 where no nozzle mechanical or fluid resonance requirements need be met.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to provide a filter as a part of a nozzle assembly of a drop writing system to prevent very small particles in the writing fluid reaching the nozzle orifice.

In accordance with the above, it is an object of the present invention to provide a filter in the nozzle assembly yet not to affect the fluid resonance of the nozzle.

It is an overall object of the present invention in accordance with the above to provide an economically manufacturable and easily serviceable filter subassembly for a nozzle assembly in a drop writing system.

The foregoing and other objects-of the invention are achieved by locating the filter in a fluid entrance chamber that has a predetermined diameter ratio to that of the nozzle flow cavity. The fluid resonance of the nozzle flow cavity acting like a closed pipe, is substantially unchanged by the filter, if the filter spans an entrance chamber cross sectional area in which the ratio of the diameter of the flow cavity cross section to the diameter of the entrance chamber cross section is about 0.45 or less. The term diameter is used in the general sense to describe the present invention. It is not limited to measurement of a circular cross section, but is instead intended to include measurement of a minimum distance along a line passing substantially through the center and subtended by the periphery of any cross section.

THE DRAWINGS DETAILED DESCRIPTION OF THE INVENTION Turning to the drawings and FIG. 1 in particular, thereshown is an ink flow system 10 for a drop writing apparatus. A reservoir of ink is maintained in an ink supply tank 11 which supplies ink to a pump 12 and receives ink from a waste ink trough 14. The pump 12 forces ink under pressure to a regulator 15 which has a solenoid valve 16 at its outlet, operative either to direct ink to a supply line 17 communicating with a print head 18 or to direct ink to a bypass line 19 for returning the ink to the supply tank 11. The print head 18 is adapted to direct a stream of droplets 20 either to the surface of a writing medium 21 or to the waste trough 14. In a well-known manner, the print head 18 traverses across the surface of the writing medium 21 to print the desired forms on the medium. A filter 22 is provided in the supply line 17 between the pump 12 and the print head 18. This is the approach which has been used in the prior art. The filter 22 includes a pair of filters in line with the flow system, a first stage coarse filter and a second stage fine filter. In one instance the latter had a porosity so as to be able to screen out particles of 40 microns or greater from the ink flowing through.

Because the print head 18 moves transversely while a connection is maintained with a supply of ink under pressure, a flexible tube 24 is included in the line 17 to accommodate the variation in length.

Turning to FIG. 2, thereshown is a cross section through the print head 18 showing the nozzle details. A longitudinally extending nozzle body 25 is provided with a generally centrally located nozzle cavity 26 extending through the length of the body 25. At the forward end of the nozzle cavity there is a restriction 27 defining an orifice 28 through which the ink issues under pressure. In practice, the restriction 27 may take the form of a jewel and the orifice 28 may have a diameter of the order of 12 to 75 microns. In one practical embodiment the cavity 26 has a diameter of 0.037 inch. The opposite end of the nozzle cavity is provided with a fluid entrance opening 28a.

The nozzle body 25 also has respective threaded portions 30, 31 at the forward and rear ends thereof. The

threaded portion receives a nut 32 which is adapted to be tightened against a set of piezoelectric crystals 34, 35, squeezing them against the integral nozzle body retainer portion 36. I

As is well-known in the art, the piezoelectric elements 34, 35 are polarized so that when electrical energy is applied thereto through wires 37a, 37b the axial dimensions of the crystals increase to apply a constriction to the nozzle body 25. These constrictions are transmitted to the fluid in the cavity 26. In this manner perturbations are effected in the fluid issuing under pressure from the orifice 28 causing them to break up into droplets of predetermined spacing as identified by reference character 20 in FIG. I. The nozzle body 25, the nut 32 and the piezoelectric elements 34, 35 are enclosed within a print head housing 38.

In accordance with this invention, there is provided a filter adjacent the fluid entrance opening 28a of the nozzle cavity 26.'To this end a filter 39 is mounted to span the cross sectional area of an entrance chamber 40 provided, in the present instance, in a filter connector union 41. Even though the filter is shown as having a generally conical shape, it is contemplated by the present invention that it may take any of many shapes. By way of example and not as a limitation, rectangular and pyramid shapes are noted. Nor is there a requirement that the filter be mounted substantially perpendicularly to an axis of cavity 26, instead it may be skewed. The aim is to position the filter 39 so as to span the flow area of the entrance chamber, yet have the filter supported so that it does not interfere with the resonance of the nozzle cavity. The connector 41 has internal threads 41a'to engate the nozzle body portion threads 31 at its forward end. At the opposite, rear end external threads 42 are provided to engage threads 44a formed in a nut 44 which is adapted to grip the end of the flexible tube 24 to form a fluid tight seal between the tube and the connector 41. The nozzle body 25 and its associated assemblies along with the screen connector 41 are anchored to the print head housing 38 by a retaining nut 45. As herein illustrated, the retaining nut 45 takes the form of a bushing having threads 46 for engaging complementary threads in the housing 38, the bushing having a radially inwardly extending lip 48 for engaging with an annular ledge 480 provided on the connector 41. An O-ring 47 is interposed between the piezoelectric crystal anchoring nut 32 and the housing 38 in the exemplary embodiment to facilitate tightening of the nozzle assembly withinthe housing. In one practical embodiment the filter was made of a polyamide with the peripheral edges molded into the connector 41, itself formed of styrene modified polyphenolene oxide. It is within the scope of this invention to provide other means and materials for supporting a spanning filtering element having the presently contemplated porosity and positioned within a chamber.

It is a feature of the present invention that even though the filter 39 has a porosity so low that it acts substantially like a solid material to accoustical waves, the resonance of the cavity 26 is substantially unaffected by the filter 39. As is best shown in FIG. 3, the fluid resonance frequency of a given nozzle remains substantially unchanged with a filter, plot F, or without a filter, plot NF, as long as the ratio of the diameter D, of the nozzle cavity cross sectional area to the diameter D of the entrance chamber cross sectional area has a ratio of about 0.45 or less. At points clearly above the 0.45 ratio point the resonance of the nozzle cavity is different when a filter or screen is positioned to span the cross sectional area of the entrance chamber, as compared to not having a filterspanning the cross sectional area.

As has been explained previously, diameter is used in the broad sense to measure a minimum distance through the center of the cross section so as to include within the scope of the present invention all the various cross sectional shapes which may be employed for the cavity 26 and chamber 40. It is contemplated that the entrance chamber cross section center is substantially coincident with the nozzle cavity axis.

It is clear from the foregoing that the present invention allows location of a filter having a porosity of the order of 40 microns in close proximity to the nozzle orifice of a drop writing nozzle. Because the orifice is relatively small of the order of l2 to microns, relatively tiny particles can cause clogging. It is contemplated that the filter 39 must be selected to have a microscopic porosity so as to be able to trap particles of about one-half the diameter of the orifice. Thus, what may be considered relatively clean conditions in an ordinary flow system can be troublesome conditions in the present drop writing system. By being able'to locate the filter which can remove micro particles that would cause clogging of the orifice relatively near to the orifice, the particles can be stopped before they reach the orifice and cause clogging. Also, by providing a structure which allows the easy disconnection of the filter from the nozzle, if a clogging of the filter occurs, it can be removed and a clean one inserted and the system is ready to operate. This is particularly desirable where the drop writing system is in the field and extremely controlled environments are not available to service the nozzle. Thus any particles which may be trapped in the system between the filter 22 and the printhead 18 need not be a cause for concern because they will be caught by the filter adjacent the nozzle. It has been found that the present invention has greatly increased the reliability of a drop writing system.

I claim as my invention:

1. In the fluid flow system of a drop writing apparatus having a print head including a nozzle body having a flow cavity with a fluid ejection orifice at one end and a fluid entrance opening at the opposite end, said cavity having selected dimensions including a predetermined diameter and length for operation at a fluid resonance frequency, the combination comprising a fluid entrance chamber cross section of a predetermined diameter in fluid communication with the fluid entrance opening of said flow cavity and a filter spanning said entrance chamber cross section, the ratio of said fluid cavity diameter to said fluid entrance chamber diameter being about 0.45 or less.

a 2. The combination of claim 1 including a hollow connector interposed between a fluid source line and the nozzle flow cavity for carrying said filter.

3. The combination of claim 2 wherein the nozzle body and said connector are provided with interfltting means adapted to permit manual connection and disconnection of said filter carrying connector relative to the nozzle body and the fluid source line.

4. In a method for filtering ink flowing through a nozzle of a drop writing system, the nozzle having a cavity of a selected length and diameter to operate at fluid resonance and the filter capable of removing particles of a predetermined size, the steps comprising forming in the nozzle an entrance chamber of a predetermined diameter in direct fluid communication with said cavity wherein the cavity diameter and said entrance chamber diameter are in a ratio of 0.45 or less and positioning the filter to span said entrance chamber to remove particles of the predetermined size flowing toward the nozzle cavity. 

1. In the fluid flow system of a drop writing apparatus having a print head including a nozzle body having a flow cavity with a fluid ejection orifice at one end and a fluid entrance opening at the opposite end, said cavity having selected dimensions including a predetermined diameter and length for operation at a fluid resonance frequency, the combination comprising a fluid entrance chamber cross section of a predetermined diameter in fluid communication with the fluid entrance opening of said flow cavity and a filter spanning said entrance chamber cross section, the ratio of said fluid cavity diameter to said fluid entrance chamber diameter being about 0.45 or less.
 2. The combination of claim 1 including a hollow connector interposed between a fluid source line and the nozzle flow cavity for carrying said filter.
 3. The combination of claim 2 wherein the nozzle body and said connector are provided with interfitting means adapted to permit manual connection and disconnection of said filter carrying connector relative to the nozzle body and the fluid source line.
 4. In a method for filtering ink flowinG through a nozzle of a drop writing system, the nozzle having a cavity of a selected length and diameter to operate at fluid resonance and the filter capable of removing particles of a predetermined size, the steps comprising forming in the nozzle an entrance chamber of a predetermined diameter in direct fluid communication with said cavity wherein the cavity diameter and said entrance chamber diameter are in a ratio of 0.45 or less and positioning the filter to span said entrance chamber to remove particles of the predetermined size flowing toward the nozzle cavity. 